1. Field of the Invention
The invention relates to a constant velocity universal joint for use in power transmission devices in motor vehicles and various industrial machines. In particular, the invention relates to a tripod type constant velocity universal joint.
2. Description of the Related Art
Tripod type constant velocity universal joints are used, for example, as an element in a power transmission device for transmitting rotational power from a car engine to wheels.
In general, a tripod type constant velocity universal joint is chiefly composed of an outer joint member and a tripod member. The outer joint member has an inner periphery provided with three axially-extending track grooves. Each of the track grooves has axial roller guideways on both sides. The tripod member is provided with three radially-projecting trunnions. A roller is rotatably arranged on each of the trunnions. The trunnions of the tripod member and the roller guideways of the outer joint member engage with each other in the direction of rotation via the rollers so that rotational torque is transmitted from a drive side to a driven side at constant velocity. The individual rollers rotate about the trunnions and roll on the roller guideways as well, absorbing relative axial displacements and angular displacements between the outer joint member and the tripod member. In the meantime, also absorbed are axial displacements of the individual trunnions to the roller guideways, the axial displacements resulting from phase changes in the direction of rotation when the outer joint member and the tripod member transmit rotational torque with some operating angle therebetween.
Some tripod type constant velocity universal joints have the rollers mounted on cylindrical outer peripheries of their trunnions via a plurality of needle rollers. When an outer joint member and a tripod member transmit rotational torque with an operating angle, however, the trunnions tilt to make the rollers and the respective roller guideways oblique to each other. This produces a slide therebetween, giving rise to a problem that resistance here hampers the smooth rolling of the rollers and thereby increases induced thrust. Moreover, there is another problem that the resistance between the rollers and the respective roller guideways increases the slide resistance to axial relative displacements between the outer joint member and the tripod member. Such induced thrust and slide resistance contribute to the production of vibrations and noises from a car body, affecting the Noise Vibration Harshness (hereinafter referred to as xe2x80x9cNVHxe2x80x9d) performances of the motor vehicle. Typical automotive NVH phenomena associated with such induced thrust and slide resistance include the rolling of a moving car body and the vibrations of a car idling with its automatic transmission in the drive or D range, respectively. The essence of solution to the automotive NVH problems consists in reducing the induced thrust and slide resistance in the joint. In general, induced thrusts and slide resistances in a joint tend to depend on operating angle of the joint. This tendency leads to a design limitation of prohibiting greater operating angles when, for example, a constant velocity universal joint is applied to an automotive drive shaft. Accordingly, reduction and stabilization of the induced thrust and slide resistance are also desired for the sake of enhanced design flexibility of portions around the car axles.
Conventionally, to eliminate the oblique states between the rollers and the roller guideways to lower the induced thrust and slide resistance, there have been proposed and put into practical use a variety of tripod type constant velocity universal joints that comprise mechanisms (roller assemblies) for allowing tilting movements of the rollers with respect to the trunnions. Among the known tripod type constant velocity universal joints of this kind is a constitution comprising outer rollers to be guided by the roller guideways and inner rollers rotatably supported by the outer peripheries of the trunnions via a plurality of needle rollers. This constitution is then broadly divided into the following modes a) to d).
a) The outer rollers are provided with outer peripheries of convex spherical shape (including both a xe2x80x9cperfect spherical surface,xe2x80x9d having its center of curvature on the trunnion axis, and a so-called xe2x80x9ctorus surface,xe2x80x9d having its center of curvature off the trunnion axis toward the outer-diameter side) and inner peripheries of cylindrical shape, and the inner rollers are provided with outer peripheries of convex spherical shape, so that slides between the cylindrical inner peripheries of the outer rollers and the convex-spherical outer peripheries of the inner rollers permit the tilting movements of the outer rollers.
b) The outer rollers are provided with outer peripheries of convex spherical shape (including both a perfect spherical surface and a torus surface) and inner peripheries shaped so as to make line contact with outer peripheries of the inner rollers, and the inner rollers are provided with the outer peripheries of convex spherical shape, so that slides between the inner peripheries of the outer rollers and the convex-spherical outer peripheries of the inner rollers permit the tilting movements of the outer rollers. Besides, the inner peripheries of the outer rollers are shaped so that load components toward the trunnion extremities are created at the contact positions with the outer peripheries of the inner rollers.
c) The roller guideways are provided with flat surfaces, the outer rollers are with outer peripheries of cylindrical shape and inner peripheries of concave spherical shape, and the inner rollers are with outer peripheries of convex spherical shape, so that slides between the concave-spherical inner peripheries of the outer rollers and the convex-spherical outer peripheries of the inner rollers permit the tilting movements of the outer rollers.
d) In addition to the constitution c) above, the roller guideways and the axes of the trunnions are configured not to be parallel to each other at an operating angle of 0xc2x0.
Also known as a tripod type constant velocity universal joint of this kind is the constitution e) in which: the outer peripheries of the trunnions are shaped into a convex spherical surface (a perfect spherical surface having the center of curvature on the trunnion axis); the rollers are mounted onto support rings via a plurality of needle rollers to constitute roller assemblies; and cylindrical inner peripheries of the support rings are fitted to the convex-spherical outer peripheries of the trunnions. The plurality of needle rollers are arranged without any retainers, or in a so-called full complement state. According to this constitution, slides between the cylindrical inner peripheries of the support rings and the convex-spherical outer peripheries of the trunnions allow the tilting movements of the roller assemblies including the rollers.
In constant velocity universal joints comprising roller assemblies of this type, axial relative movements of the rollers and the support rings are restricted from both sides by engaging means so that the roller assemblies are secured in their unity as assembled articles. On the other hand, when a constant velocity universal joint of this kind transmits rotational torque at an operating angle, tilting movements and axial movements of the roller assemblies with respect to the trunnions produce slides between the inner peripheries of the support rings and the outer peripheries of the trunnions. Then, the sliding frictional forces therein cause axial repetitive loads (hereinafter, simply referred to as xe2x80x9caxial loadsxe2x80x9d) onto the engaging means, along the axial directions of the rollers and the support rings. Hence, the engaging means require such strengths as to stand the axial loads (strengths against bending fatigue, cracking fatigue, and the like). Besides, the engaging means make sliding contact with the end faces of the rollers and/or the support rings and, in the cases where the rollers are rotatably supported by the support rings via needle rollers, even with the end faces of the needle rollers. This brings about another problem of the fatigue life of those contact surfaces.
In view of the foregoing, it is an object of the present invention to make further reduction and stabilization of the induced thrust and slide resistance in this kind of tripod type constant velocity universal joint.
Another object of the present invention is to make improvements on this kind of tripod type constant velocity universal joint in the rolling fatigue life of the individual component parts and their strengths against torsional fatigue, crack, and the like, so as to provide a tripod type constant velocity universal joint of superior durability and strengths while maintaining current dimensions, and provide a tripod type constant velocity universal joint of more compact configuration while securing durability and strengths equivalent to or higher than those of existing products.
Still another object of the present invention is to make improvements on a tripod type constant velocity universal joint comprising roller assemblies as described above in the fatigue strength of the engaging means, especially of the engaging rings to be attached to the rollers/support rings, against axial loads and in the fatigue life of their contact surfaces, so as to provide a tripod type constant velocity universal joint of superior durability and strengths while maintaining its current dimensions, and provide a tripod type constant velocity universal joint of more compact configuration while securing durability and strengths equivalent to or higher than those of existing products.
To achieve the foregoing objects, the present invention provides a constant velocity universal joint comprising: an outer joint member having three track grooves each having circumferentially-opposed roller guideways; a tripod member having three radially-projecting trunnions; a roller inserted in each of the track grooves; and a support ring mounted on each of the trunnions to support the roller rotatably, the roller being movable in axial directions of the outer joint member along the roller guideway, wherein the outer periphery of the roller is a partial spherical surface having the center of curvature on the trunnion axis, and the roller guideways form partial cylindrical surfaces parallel to the axis of the outer joint member, so that the roller is capable of tilting in the track groove.
In the constitution described above, the inner periphery of the support ring is shaped arcuate and convex in section. The outer periphery of each of the trunnions is shaped straight in longitudinal section, and formed in cross section so as to make contact with the inner periphery of the support ring in a direction perpendicular to the axis of the joint and create a clearance with the inner periphery of the support ring in an axial direction of the joint. The cross-sectional configuration of a trunnion such as makes contact with the inner periphery of the support ring in a direction perpendicular to the axis of the joint and creates a clearance with the inner periphery of the support ring in an axial direction of the joint translates into that the faces opposed to each other in the axial direction of the tripod member retreat toward each other, i.e., to smaller diameters than the diameter of an imaginary cylindrical surface. Concrete examples thereof include an ellipse. For the sake of absorbing the tilt of the trunnions ascribable to nutations peculiar to tripod type constant velocity universal joints, the radius of curvature to the convex arcs of the support rings preferably has a value that allows the trunnions to make a tilt of the order of 2-3xc2x0.
The trunnions may be formed to have a cross section of generally elliptic shape with the major axis perpendicular to the axis of the joint. The generally elliptic shape is not limited to literal ellipses, and is intended to include those generally referred to as ovals and the like. More specifically, the configurations can be adopted for the cross sections of the trunnions and the inner peripheries of the support rings so that the contact pressures against the support rings are relaxed and the trunnions are prevented from a strength drop. Besides, as long as the operating angle falls within a predetermined angle range, the trunnions can tilt without inclining the support rings. This prevents the rollers from inclination and allows the rollers to roll smoothly on the roller guideways. There is provided no ribs which have sometimes been arranged on the track grooves in the outer joint member with an aim to restrain the inclination of the rollers. The omission of the ribs not only reduces the outer joint member in weight and simplifies the machining thereto, but eliminates slide resistance resulting from the slide contacts between the rollers and the ribs. This consequently achieves further reductions in slide resistance and induced thrust.
The outer periphery of each of the trunnions and the inner periphery of the support ring may create a clearance of 0.001a or greater in a circumferential direction of the joint, where a is the semimajor axis of the generally elliptic cross section of the trunnion. Such clearances can well absorb the tilt of the trunnions resulting from the nutations of the tripod member which are peculiar to tripod type constant velocity universal joints. This absorption then removes the factors responsible for the inclinations of the roller assemblies in the joint""s cross section.
The support rings may have a cylindrical inner periphery. Since the support rings having the cylindrical inner peripheries are mounted to the trunnions"" outer peripheries having a generally elliptic cross section, they make line contacts along the axial direction of the trunnions with an advantageous reduction in surface pressure. In this case, the trunnions are limited in possible tilt angle to the support rings. Here, the rollers are configured to be tiltable inside the track grooves as described above, and hence the rollers tilt at greater operating angles while moving along the track grooves.
The trunnions may have a cylindrical outer periphery, and the generatrix of the inner peripheries of the support rings may comprise a convex arc at the center. Since the spherical support rings are mounted on the trunnions"" cylindrical outer peripheries, they make line contact along the circumferential directions of the trunnions with an advantageous reduction in surface pressure. Again, the trunnions are limited in possible tilt angle to the support rings. The rollers are configured to be tiltable inside the track grooves as described above, and hence the rollers tilt at greater operating angles while moving along the track grooves.
In the constitutions described above, a plurality of rolling elements may be interposed between the support rings and the rollers to allow relative rotations of the support rings and the rollers. The rolling elements may be needle rollers.
According to the present invention, when the joint transmits torque with an operating angle, the tilt of the trunnions can be absorbed into the tilt of the rollers. This contributes to a reduction in slide resistance and, finally, to a reduction in induced thrust. The constant velocity universal joints of the present invention are particularly applicable to a motor vehicle""s drive shaft. This application can contribute to improvements in automotive NVH performances that depend on slide resistance and induced thrust, thereby increasing design flexibility of portions around the car axles.
To achieve the foregoing objects, the present invention also provides a constant velocity universal joint comprising: an outer joint member having three track grooves each having circumferentially-opposed roller guideways; a tripod member having three radially-projecting trunnions; a roller inserted in each of the track grooves; and a support ring mounted on each of the trunnions to support the roller rotatably, the roller being movable in axial directions of the outer joint member along the roller guideways, wherein: the support ring has a cylindrical inner periphery; and the outer periphery of each of the trunnions is curved in longitudinal section, and formed in cross section so as to make contact with the inner periphery of the support ring in a direction perpendicular to the axis of the joint and create a clearance with the inner periphery of the support ring in an axial direction of the joint.
The cross-sectional configuration of a trunnion such as makes contact with the inner periphery of the support ring in a direction perpendicular to the axis of the joint and creates a clearance with the inner periphery of the support ring in an axial direction of the joint translates into that the faces opposed to each other in the axial direction of the tripod member retreat toward each other, i.e., to smaller diameters than the diameter of an imaginary cylindrical surface. Concrete examples thereof include an ellipse.
Due to the changes in cross section from the conventional circular shape to the configuration described above, the trunnions can tilt with respect to the outer joint member without changing the orientations of the roller assemblies when the joint operates with an operating angle. Besides, the contacting ellipses of the support rings with the outer peripheries of the trunnions approach from oblong ellipses to points in shape. This reduces the friction moments which act to tilt the roller assemblies. As a result, the roller assemblies are always stabilized in orientation, whereby the rollers are maintained parallel to the roller guideways for smooth rolling. This smooth rolling contributes to a reduction in slide resistance and, finally, to a reduction in induced thrust. Moreover, there is an advantage that the trunnions improve in flexural strength due to increased section moduli at the bottom portions of the trunnions. In this connection, the inner peripheries of the support rings need not be cylindrical over the entire lengths thereof. They may be formed cylindrical only at their centers for making contact with the trunnions, and provided with relief portions on both sides so as to avoid interference when the trunnions tilt.
The roller assemblies are interposed between the trunnions and the outer joint member for the sake of torque transmission. In constant velocity universal joints of this kind, the transmission direction of torque is always perpendicular to the axis of the joint. Therefore, as long as they contact in the transmission direction of torque, the trunnions and the support rings can perform torque transmission without trouble even when they have clearances therebetween in the axial directions of the joint.
In the above-described constitution, the trunnions may be formed to have a cross section of elliptic shape with the major axis perpendicular to the axis of the joint. The generally elliptic shape here is not limited to literal ellipses, and is intended to include those generally referred to as ovals and the like.
More specifically, the trunnions can adopt such cross-sectional configurations so that the contact pressures against the support rings are relaxed and the trunnions are prevented from a strength drop. Besides, the trunnions can tilt without inclining the support rings. This prevents the rollers from inclination and allows the rollers to roll smoothly on the roller guideways. As a result, it becomes possible to omit ribs which are sometimes arranged on the track grooves in the outer joint member with an aim to restrain the inclination of the rollers. The omission of the ribs not only reduces the outer joint member in weight and simplifies the machining thereto, but eliminates slide resistance resulting from the slide contacts between the rollers and the ribs. As a result, further reductions in slide resistance and induced thrust are achieved.
The curve to the longitudinal sections of the trunnions may have a radius of curvature in the range of 1.1a and 8.7a. This makes it possible to absorb the tilt of the trunnions resulting from the nutations of the tripod member which are peculiar to tripod type constant velocity universal joints. This absorption removes the factors responsible for the inclinations of the roller assemblies in the joint""s cross section, and thereby contributes to improved NVH performances of motor vehicles.
The outer periphery of each of the trunnions may be ground only at a predetermined region including an area for making contact with the support ring. In contemplation of machining errors and the like, the predetermined region is preferably determined to be somewhat wider than the contact area. The remaining portions other than the predetermined region may be left forge-finished without any grinding. This allows a reduction in machining time and a cut in costs.
The outer periphery of the roller and the roller guideways in the outer joint member may make angular contact with each other. The angular contact between the roller and the roller guideways makes the roller less prone to vibrate, further stabilizing the orientation of the roller. As a result, the roller can roll on the roller guideways with smaller resistance when moving along the axial direction of the outer joint member. The specific constitutions to establish such angular contact include a convex arcuate generatrix to the outer periphery of the roller, combined with roller guideways having a tapered or Gothic arc cross section.
In the above-described constitutions, a plurality of rolling elements can be interposed between the support rings and the rollers to allow relative rotation of the support rings and the rollers, so that the rollers can make smooth rotation around the trunnions for reduced slide resistance. The rolling elements may be needle rollers or balls.
According to the present invention, the trunnions can tilt with respect to the outer joint member without changing the orientations of the roller assemblies when the joint operates with an operating angle. Besides, the contacting ellipses of the support rings with the outer peripheries of the trunnions approach from oblong ellipses to points in shape. This reduces the friction moments which act to tilt the roller assemblies. As a result, the roller assemblies are always stabilized in orientation, whereby the rollers are maintained parallel to the roller guideways for smooth rolling. This smooth rolling contributes to a reduction in slide resistance and, finally, to a reduction in induced thrust. Moreover, there is an advantage that the trunnions improve in flexural strength because of increased section moduli at the bottom portions of the trunnions.
The constant velocity universal joints of the present invention are particularly applicable to a motor vehicle""s drive shaft. This application can contribute to improvements in automotive NVH performances that depend on slide resistance and induced thrust, thereby increasing design flexibility of portions around the car axles.
To achieve the foregoing objects, the present invention also provides a constant velocity universal joint comprising: an outer joint member having an inner periphery provided with three axial track grooves, axial roller guideways being arranged on both sides of each of the track grooves; a tripod member having three radially-projecting trunnions; and a roller assembly mounted on each of the trunnions of the tripod member, the roller assembly being capable of tilting movement with respect to the trunnion and having a roller to be guided along the roller guideways in directions parallel to the axis of the outer joint member, wherein at least one component part of the joint is limited to a predetermined range in softening resistance characteristic value (R).
The present applicant has found from a number of experiments that the durability of the component parts of the above-described constant velocity universal joint, and particularly the durability of the tripod member and the outer joint member, can be controlled accurately by using the softening resistance characteristic value R mentioned above.
Take the tripod member as an example. The factors affecting the durability of the same include rolling fatigue on the outer peripheries of the trunnions, torsional fatigue at the trunnion bottoms, and torsional fatigue in a serration portion (or spline portion). The outer peripheries of the trunnions make rolling contact with the outer peripheries of the needle rollers, or make rolling and sliding contact with the inner peripheries of the support rings in the roller assemblies, and thus have the problem of rolling fatigue. The trunnion bottoms and the serration portion undergo concentrated torsional stresses in torque transmission. This combines with the fact that these portions are usually left unground, to give rise to the problem of torsional stresses. Now, taking the outer joint member as an example, the factors affecting the durability thereof include rolling fatigue on the roller guideways in the track grooves. The roller guideways make rolling and sliding contact with the outer peripheries of the rollers, and thus have the problem of rolling fatigue. Besides, the outer joint member receives joint loads through the rollers, and hence has a problem in crack strength. Moreover, other component parts comprising the roller assemblies also have the problem of rolling fatigue at portions to make rolling contact and/or sliding contact with their mating members.
In general, it is well known that the fatigue strengths of steel material have a correlation to surface hardness. Steel materials are therefore subjected to heat treatment and the like to form hardened surface layers, so that the hardened surface layers are controlled in surface hardness for required fatigue strength. The results of the experiments made by the present applicant, however, have shown that the fatigue strengths have a closer correlation with softening resistance characteristics of a region ranging from the surface to a predetermined depth (anti-softening property of the material at some higher temperatures) than with surface hardness. Then, it has been found that the softening resistance characteristics can be properly evaluated in terms of the maximum hardness of the region within 0.5 mm in depth from a predetermined surface (softening resistance characteristic value R), and this softening resistance characteristic value R can be used as the evaluation index to the fatigue strengths. The xe2x80x9csoftening resistance characteristic value Rxe2x80x9d herein will be expressed as the maximum Vickers hardness Hv in the region within 0.5 mm in depth from the surface, of a component part that is hardened and then tempered at 200xc2x0 C.xc3x972 h. This softening resistance characteristic value R can be limited to a predetermined range to improve the rolling fatigue life of the component part and enhance its strengths against torsional fatigue and the like.
When component parts are made of steel having a carbon content of 0.15-0.40% by weight and are provided with a surface layer formed by carburizing and tempering beneath a predetermined surface, the softening resistance characteristic value R thereof can be limited to the range of 705 less than Rxe2x89xa6820, and preferably 710xe2x89xa6Rxe2x89xa6815, for desirable results.
When component parts are made of steel having a carbon content of 0.15-0.40% by weight and are provided with a surface layer formed by carbonitriding and tempering beneath a predetermined surface, the softening resistance characteristic value R thereof can also be limited to the range of 705 less than R less than 820, and preferably 710xe2x89xa6Rxe2x89xa6815, for desirable results.
When component parts are made of steel having a carbon content of 0.45-0.60% by weight and are provided with a surface layer formed by induction hardening and tempering beneath a predetermined surface, the softening resistance characteristic value R thereof can be limited to the range of 630 less than Rxe2x89xa6820, and preferably 640xe2x89xa6Rxe2x89xa6810, for desirable results.
According to the present invention, the materials of the component parts, notably of the tripod member and the outer joint member, and the properties of the surfaces and subsurfaces thereof are optimized for improvements in rolling fatigue life and in the strengths against torsional fatigue and the like. This makes it possible to provide a tripod type constant velocity universal joint of superior durability and strengths while maintaining its current dimensions, as well as to provide a tripod type constant velocity universal joint of more compact configuration while securing durability and strengths equivalent to or higher than those of existing products.
Moreover, to achieve the foregoing objects, the present invention provides a constant velocity universal joint comprising: an outer joint member having an inner periphery provided with three axial track grooves, axial roller guideways being arranged on both sides of each of the track grooves; a tripod member having three radially-projecting trunnions; and a roller assembly mounted on each of the trunnions of the tripod member, the roller assembly being capable of tilting movement with respect to the trunnion and having a roller to be guided along the roller guideways in directions parallel to the axis of the outer joint member, wherein at least one component part of the joint has a surface portion having a residual austenite content xcex3R (vol %) in the range of 20xe2x89xa6xcex3Rxe2x89xa640.
Generally, among typical fatigues on rolling contact surfaces is flaking (fatigue exfoliation). More specifically, it is known that contact surfaces subjected to repeated loads from rolling movements generate cracks in their rolling portions, and these cracks develop to flaking so that the rolling fatigue life is reached. Here, a number of experiments and experiences have shown that the cracks that originate flaking often occur at portions somewhat inside the contact surfaces. It has been also found that, under such condition that metal wear chips and other foreign matters easily get into the lubricant, the contact surfaces develop damage similar to the original flaking and reach their rolling fatigue life because of exfoliation originating from foreign-matter-biting indentations, peeling and smearing due to insufficient lubricating oil films, and cracks originating therefrom (surface-origin type damage). In the latter case, the rolling fatigue life of the contact surfaces becomes shorter than under lubrication conditions with clean lubricants.
Meanwhile, in constant velocity universal joints of this kind, the component parts have contact surfaces of greater surface roughness as compared with those of ordinary rolling bearings. In addition, when the rollers make tilting movements with respect to the trunnions, a slide occurs in the contact portions between the support rings of the roller assemblies and the trunnions, or in the contact portions between the inner and outer rollers of the roller assemblies. The contact portions consequently produce wear chips, which get into the lubricant and are bitten between the contact surfaces, contributing to the generation of indentations and the hindered formation of lubricating oil films with easier occurrence of the surface-origin type damage mentioned above.
According to the present invention, at least one of the component parts is provided with a surface layer having a residual austenite content xcex3R (vol %) limited to the range of 20xe2x89xa6xcex3Rxe2x89xa640. Therefore, the surface layer improves in crack sensitivity so that the surface-origin type damage described above become harder to occur. Here are the reasons for this. That is, residual austenite is relatively low in hardness (e.g. Hv 300 or so, though depending on the carbon content of the material). Therefore, even if indentations are formed in the contact surface due to the biting of foreign matters, austenite particles distributed in the surface portion facilitate elastic deformation around the indentations, and thereby relax the stress concentration on the surface layer and delay the propagation of crack. Besides, because of the deformation energy, the residual austenite undergoes a martensite transformation for hardening. Therefore, providing the surface layer with an appropriate amount of residual austenite can improve the surface layer in crack sensitivity so that the production of the surface-origin type damage described above is suppressed for enhanced rolling fatigue life. Residual austenite contents xcex3R of a surface layer below 20% by volume cannot make a sufficient improvement to the crack sensitivity of the surface layer. On the other hand, residual austenite contents xcex3R of a surface layer above 40% by volume promise no further improvement to the crack sensitivity but cause a drop in surface hardness, thereby decreasing the rolling fatigue life contrarily. Accordingly, the surface portion is preferably set within the range of 20xe2x89xa6xcex3Rxe2x89xa640 in residual austenite content xcex3R (vol %). Incidentally, the surface layer in the present invention need only be formed at least beneath the contact surface of the component part. This includes the constitution that a surface layer is formed only beneath the contact surface, the constitution that surface layers are formed beneath the contact surface and surfaces adjacent thereto, and the constitution that surface layers are formed beneath the entire surfaces of the component part.
For example, at least one of the outer joint member, the tripod member, and the component parts of the roller assemblies may be formed of steel having a carbon content of 0.15-0.40% by weight, and provided with a carburized-and-tempered surface portion (carburized layer) or a carbonitrided-and-tempered surface portion (carbonitrided layer). Here, the residual austenite content xcex3R (vol %) of the surface portion is limited to the range of 20xe2x89xa6xcex3Rxe2x89xa640. According to this constitution, the surface portion of that particular component part improves in crack sensitivity to have a structure of superior durability against rolling fatigue, while the core portion thereof forms a structure having toughness. As a result, the component part combines long rolling fatigue life with crack strength and the like. This effect is particularly significant in the constitutions having carbonitrided-and-tempered surface portions (carbonitrided layers). More specifically, when nitrogen is combined into a surface layer under appropriate conditions, the residual austenite and the martensite matrix become stable toward heat due to the intrusion of nitrogen. This means a structure less prone to thermal changes, with higher resistance against rolling fatigue and higher strengths against cracks and the like. The trunnion bottoms and the serration portion of the tripod member undergo concentrated torsional stresses in torque transmission, and these portions are usually left unground. As a result, there occurs the problem of torsional stresses. Nevertheless, the formation of carbonitrided layers improves hardenability, whereby these portions are increased in surface hardness and improved in torsional fatigue strength as well.
For example, at least one of the parts constituting the roller assemblies may be made of steel having a carbon content of 0.95-1.10% by weight, and provided with a surface layer of nitride layer (layer having more solid solution of nitrogen) formed by nitriding and tempering beneath its contact surface. Here, the residual austenite content xcex3R (vol %) of the surface layer is limited to the range of 20xe2x89xa6xcex3Rxe2x89xa640. As in the constitutions described above, the surface layer of this component part improves in crack sensitivity to have a structure of superior rolling fatigue strength. At the same time, the hardening uniformly extends to the inside, advantageously decreasing deformation under high load. As a result, this component part combines longer rolling fatigue life with higher load deformation resistance and the like.
In the constitutions described above, the softening resistance characteristic value R of at least either of the outer joint member and the tripod member is desirably limited to the range of 705 less than Rxe2x89xa6820, and preferably 710xe2x89xa6Rxe2x89xa6815, for the reasons stated previously.
According to the present invention, the materials of the component parts and the properties of the surface layers are optimized for improvements of rolling fatigue life, notably of the resistance against surface-origin type damage resulting from the biting of wear chips and other foreign matters. This makes it possible to provide a tripod type constant velocity universal joint of superior durability and strengths while maintaining its current dimensions, as well as to provide a tripod type constant velocity universal joint of more compact configuration while securing durability and strengths equivalent to or higher than those of existing products.
Furthermore, to achieve the foregoing objects, the present invention provides a constant velocity universal joint comprising: an outer joint member having an inner periphery provided with three axial track grooves, axial roller guideways being arranged on both sides of each of the track grooves; a tripod member having three radially-projecting trunnions; and a roller assembly mounted on each of the trunnions of the tripod member, the roller assembly being capable of tilting movement with respect to the trunnion and having a roller to be guided along the roller guideways in directions parallel to the axis of the outer joint member, wherein at least one component part of the jopint has a surface portion containing a structure in which carbide is distributed into a martensite matrix. This constitution includes those in which only the surface layer has the above-described structure and those in which the structure extends from the surface to the inside.
Generally, among typical fatigues on rolling contact surfaces is flaking (fatigue exfoliation). More specifically, it is known that contact surfaces subjected to repeated loads from rolling movements generate cracks in their rolling portions, and these cracks develop to flaking so that the rolling fatigue life is reached. Here, a number of experiments and experiences have shown that the cracks that originate flaking often occur at portions somewhat inside the contact surfaces. It has been also found that, under such condition that metal wear chips and other foreign matters easily get into the lubricant, the contact surfaces develop damage similar to the original flaking and reach their rolling fatigue life because of exfoliation originating from foreign-matter-biting indentations, peeling and smearing due to insufficient lubricating oil films, and cracks originating therefrom (surface-origin type damage). In the latter case, the rolling fatigue life of the contact surfaces becomes shorter than under lubrication conditions with clean lubricants.
Meanwhile, in constant velocity universal joints of this kind, the component parts have contact surfaces of greater surface roughness as compared with those of ordinary rolling bearings. In addition, when the rollers make tilting movements with respect to the trunnions, a slide occurs in the contact portions between the support rings of the roller assemblies and the trunnions, or in the contact portions between the inner and outer rollers of the roller assemblies. The contact portions consequently produce wear chips, which get into the lubricant and are bitten between the contact surfaces, contributing to the generation of indentations and the hindered formation of lubricating oil films with easier occurrence of the surface-origin type damage mentioned above.
According to the present,invention, at least the surface portion of the component part is provided with a structure in which carbide is distributed in a martensite matrix. Therefore, the surface hardness increases so that contact surfaces improve in wear resistance to suppress flaking. At the same time, foreign-matter-biting indentations become hard to occur, whereby the aforementioned surface-origin type damage are suppressed as well. This means improved rolling fatigue life of the contact surfaces.
The above-mentioned structure may be formed by making the component part in steel material having a carbon content of 0.80% by weight or higher, e.g., in high carbon chrome steel, and subjecting the same to dip quenching and tempering. According to this constitution, the contact surfaces show higher resistance against flaking and surface-origin type damage. Besides, the hardening uniformly extends to the inside, thereby reducing deformation under high load. As a result, that particular component part combines longer rolling fatigue life with load deformation resistance and the like. The high carbon chrome steel may use bearing steels such as SUJ1, SUJ2, SUJ3, SUJ4, and SUJ5.
The above-mentioned structure may also be formed by making the component part in steel material having a carbon content of 0.15-0.40% by weight, e.g., in steel for carburization, and subjecting the same to heavy carburizing and tempering. The heavy carburizing here is a process for increasing the content of the C solid solution in the matrix of the surface portion to, for example, 1.5-4.0% by weight. The carbon content of the base material, on which the core hardness depends, is preferably set within the range of 0.15-0.40% by weight for the sake of fatigue strengths. When the base metal has a carbon content below 0.15% by weight, the carburizing requires longer time and the core portion falls short of hardness as well. On the other hand, carbon contents above 0.4% by weight increase the core hardness, which significantly lowers toughness as well as increases distortion. According to this constitution, the contact surfaces show higher resistance against flaking and surface-origin type damage, while the core portion forms a structure having toughness. As a result, that particular component part combines longer rolling fatigue life with crack strength and the like. The steel for carburization may use SCr415, SCr420, SCr430, SCr435, SCr440, SCM415, SCM420, SCM430, SCM435, SCM440, SNCM220, SNCM415, SNCM420, SNCM815, and the like. The carburizing may adopt gas carburizing or plasma carburizing. In the case of gas carburizing, the carbon potential of the carburizing gas is increased to 1.5-4.0% by weight or higher for heavy carburizing. The plasma carburizing is a process in which plasma discharge of direct-current high voltage is generated between both electrodes in a vacuum through the medium of C in the carburizing gas, with the furnace body as the positive electrode and the article to be processed as the negative electrode, so that C is ionized (C+) and intruded into the matrix in the surface portion of the article. The plasma carburizing, as a carburization under non-equilibrium, can obtain a surface portion of higher C concentration in a shorter time as compared with the gas carburizing. Besides, the plasma carburizing can provide a uniform concentration distribution, and thus has an advantage that an appropriate amount of carbide can be uniformly deposited in the surface portion. In this connection, when the plasma carburizing is adopted, it is preferable that the Mo and Cr contents of the steel for carburization be made higher than usual.
The deposition of carbide in the martensite matrix as described above allows the contact surfaces to have a surface hardness of HRC 60-68, or preferably HRC 63-68. The term xe2x80x9cHRCxe2x80x9d herein represents C scale in Rockwell hardness. Surface hardnesses of the contact surfaces below HRC 60 will not lead to an improvement in rolling fatigue life, whereas those equal to or lower than HRC 68 are preferable in consideration of toughness.
According to the present invention, the materials of the component parts, or at least the structures of the surface portions are optimized for improvements in rolling fatigue life, crack strength, the like. This makes it possible to provide a tripod type constant velocity universal joint of superior durability and strengths while maintaining its current dimensions, as well as to provide a tripod type constant velocity universal joint of more compact configuration while securing durability and strengths equivalent to or higher than those of existing products.
The constant velocity universal joints according to the inventions discussed above may use roller assemblies each including the roller to be guided by the roller guideways and a support ring mounted to the outer periphery of the trunnion so as to support the roller rotatably, wherein: the inner periphery of the support ring is shaped arcuate and convex in section; and the outer periphery of the trunnion is shaped straight in longitudinal section, and formed in cross section so as to make contact with the inner periphery of the support ring in a direction perpendicular to the axis of the joint and create a clearance with the inner periphery of the support ring in an axial direction of the joint. In this constitution, it is the roller assemblies including the roller and the support ring unitarily that make tilting movements with respect to the trunnions. Here, the tilting movements refer to the tilts the axes of the support rings and rollers make with respect to the axes of the trunnions, within the planes containing the axes of the trunnions. The cross-sectional configuration of a trunnion such as makes contact with the inner periphery of the support ring in a direction perpendicular to the axis of the joint and creates a clearance with the inner periphery of the support ring in an axial direction of the joint translates into that the faces opposed to each other in the axial direction of the tripod member retreat toward each other, i.e., to smaller diameters than the diameter of an imaginary cylindrical surface. Concrete examples thereof include general elliptic shapes. The xe2x80x9cgeneral elliptic shapesxe2x80x9d include those generally referred to as ovals and the like, aside from literal ellipses.
Due to the changes in cross section from the conventional circular shape to the configuration described above, the trunnions can tilt with respect to the outer joint member without changing the orientations of the roller assemblies when the joint operates with an operating angle. Besides, the contacting ellipses of the support rings with the outer peripheries of the trunnions approach from oblong ellipses to points in shape. This reduces the friction moments which act to tilt the roller assemblies. As a result, the roller assemblies are stabilized in orientation, whereby the rollers are maintained parallel to the roller guideways for smooth rolling. This smooth rolling contributes to a reduction in slide resistance and, finally, to a reduction in induced thrust.
The roller assemblies are interposed between the trunnions and the outer joint member for the sake of torque transmission. In constant velocity universal joints of this kind, the transmission direction of torque is always perpendicular to the axis of the joint. Therefore, as long as they contact in the transmission direction of torque, the trunnions and the support rings can perform torque transmission without trouble even when they have clearances therebetween in the axial directions of the joint.
In the constitutions described above, the generatrix to the inner peripheries of the support rings may comprise an arc portion at the center and relief portions on both sides. The arc portion preferably has such a radius of curvature as allows 2-3xc2x0 inclination of the trunnions. Additionally, a plurality of rolling elements may be interposed between the support rings and the rollers so as to allow relative rotations between the support rings and the rollers. The rolling elements may be needle rollers. Furthermore, the outer peripheries of the rollers may be formed into a spherical shape (perfect spherical surfaces or torus surfaces) so that the spherical outer peripheries of the rollers and the roller guideways of the outer joint member make angular contacts with each other. The angular contacts between the rollers and the roller guideways make the rollers less prone to vibrate, thereby stabilizing the orientation of the rollers. As a result, the rollers can roll on the roller guideways with smaller resistance when moving along the axial direction of the outer joint member. The specific configurations to establish such angular contacts include tapered or Gothic-arc cross sections of the roller guideways.
Now, in the constant velocity universal joint of the above-described constitution, the contact surface pressures between the outer peripheries of the trunnions and the inner peripheries of the support rings are higher than in the other constitutions. Therefore, the outer peripheries of the trunnions tend to have a shorter rolling fatigue life. In addition, stresses are concentrated on the bottom portions of the trunnions more easily than in the other constitutions, and hence the bottom portions tend to have lower fatigue strengths. Accordingly, it is particularly effective in the constant velocity universal joint of this constitution to confine the softening resistance characteristic values R of the outer peripheries and bottom surfaces of the trunnions to a predetermined range so that the outer peripheries are enhanced in rolling fatigue life and the bottom portions are enhanced in torsional fatigue strength and other strengths as described above.
Moreover, the constant velocity universal joints according to the inventions described above may use roller assemblies each including the roller to be guided by the roller guideways and a support ring fitted on the outer periphery of the trunnion to support the roller rotatably, wherein: the trunnion has a convex-spherical outer periphery; and the support ring has a cylindrical or conic inner periphery. In this constitution, the roller assemblies including the roller and the support ring unitarily make tilting movements with respect to the trunnions.
Furthermore, the constant velocity universal joints according to the inventions described above may use roller assemblies each including an outer roller to be guided by the roller guideways, and an inner roller rotatably supported by the trunnion as well as fitted to the inner periphery of the outer roller, wherein: the inner roller has a convex-spherical outer periphery; and the outer roller has an inner periphery shaped so that a load component toward the trunnion extremity is created at a contact position with the outer periphery of the inner roller. In this constitution, the roller assemblies including the roller and the support ring make tilting movements with respect to the trunnions. Here, the tilting movements refer to the tilts the axes of the outer rollers make with respect to the axes of the trunnions, within the planes containing the axes of the trunnions.
To be more specific, the inner peripheries of the outer rollers may take a variety of configurations described in Japanese Patent Laid-Open Publication No.Hei 9-14280 by the present applicant. Namely, the configurations the inner peripheries of the outer rollers may take include the following: the form of a cone gradually contacting in diameter toward the trunnion extremity; a concave spherical surface having a generatrix whose center falls off the center of generatrix of the. trunnion""s outer periphery toward the trunnion bottom (the configuration shown in FIG. 3, Japanese Patent Laid-Open Publication No.Hei 9-14280); a convex spherical surface having a generatrix whose center falls off the center of generatrix of the trunnion""s outer periphery toward the trunnion extremity (the configuration shown in FIG. 4, Japanese Patent Laid-Open Publication No.Hei 9-14280); a composite surface of a conical tapered surface contracting in diameter toward the trunnion extremity and a convex spherical surface (the configuration shown in FIG. 5, Japanese Patent Laid-Open Publication No.Hei 9-14280); and a composite surface of a cylindrical surface and a convex spherical surface (the configuration shown in FIG. 9, Japanese Patent Laid-Open Publication No.Hei 9-14280). Nevertheless, in favor of simplified fabrication processes, the inner peripheries of the outer rollers preferably have the form of a cone gradually contracting in diameter toward the trunnion extremity. In that case, the inner peripheries of the outer rollers desirably have a tilt angle of 0.1-3xc2x0, and preferably 0.1-1xc2x0, for the sake of effective reduction and stabilization of the induced thrusts.
In the constitutions described above, many minute dimples may be formed randomly at least in the contact surfaces including the outer peripheries of the trunnions and the roller guideways. The minute dimples formed in the contact surfaces function as oil sumps to promote the formation of oil films on the contact surfaces, thereby improving the lubricity and enhancing the rolling fatigue life of the contact surfaces. For example, the minute dimples have a size of several tens of xcexcm or so, and a depth of 1 xcexcm or so. Grinding conditions to the contact surfaces can be changed to form minute dimples of arbitrary size, depth, and number. Incidentally, when it is difficult to form minute dimples selectively in the contact surfaces alone, minute dimples may also be formed in the vicinities of the contact surfaces of that component part, or over the entire surfaces.
A solid lubrication coating may be formed on the contact surfaces including the outer peripheries of the trunnions and the roller guideways, with a chemical conversion coating as an undercoating. Since the solid lubrication coating reduces the frictional resistance on the contact surfaces and improves the lubricity, the contact surfaces improve in rolling fatigue life. The chemical conversion coating to be the undercoating is formed with the objective of enhancing the solid lubrication coating in adhesion to the contact surfaces. Examples of the chemical conversion coating include a manganous phosphate coating, an iron phosphate coating, and a zinc phosphate coating. Examples of the solid lubrication film include a molybdenum disulfide coating and a PTFE coating. In this connection, the effect after the treatment depends on the pre-treatment surface roughness of the contact surfaces (base-material surfaces). It is therefore desirable that the contact surfaces be previously finished with a surface roughness of Ra 0.2-0.8, for the sake of appropriate oil-sump functions. In the cases where the selective application of coating to the contact surfaces alone is difficult, the coating may also be applied to the vicinities of the contact surfaces of those component parts, or over the entire surfaces.
Cold sulfurizing may be applied to the contact surfaces including the outer peripheries of the trunnions and the roller guideways. Sulfurizing is a surface treating method for infiltrating sulfur to the surface of steel to generate iron sulfide. The application of sulfurizing reduces the frictional resistance on the surface; therefore, the surface improves in initial conformability for enhanced rolling fatigue life and stabilized NVH performances as well. Since the cold sulfurizing is performed under such a condition as 30-40xc2x0 C.xc3x9710-30 min., no hardness drop occurs in the surface hardened layers. The effect after the treatment depends on the pre-treatment surface roughness of the contact surfaces (base-material surfaces). Thus, it is desirable that the contact surfaces be previously finished with a surface roughness of Ra 0.2-0.8 for the sake of appropriate oil-sump functions.
Moreover, to achieve the foregoing objects, the present invention provides a constant velocity universal joint comprising: an outer joint member having an inner periphery provided with three axial track grooves, axial roller guideways being arranged on both sides of each of the track grooves; a tripod member having three radially-projecting trunnions; and a roller assembly mounted on each of the trunnions of the tripod member, the roller assembly including a roller to be guided along the roller guideways in directions parallel to the axis of the outer joint member, a support ring for supporting the roller rotatably, and engaging means for retaining the roller and the support ring from both sides so as to prevent axial relative movement of the roller and the support member, the roller assembly being capable of tilting movements and axial displacements with respect to the trunnion, wherein at least either one of the engaging means has a engaging ring attached to the roller or the support ring, the engaging ring having a width W in the range of 0.5 mmxe2x89xa6Wxe2x89xa61.2 mm and a surface hardness in the range of HRC 43 to HRC 52.
Here, the constitution that xe2x80x9cat least either one of the engaging means has a engaging ring attached to the roller or the support ringxe2x80x9d includes the constitutions in which one of the engaging means is a engaging ring and the other engaging means consists of a engaging collar integrally arranged on the roller or the support-ring, and the constitutions in which both of the engaging means are engaging rings. It also covers such a constitution that at least one of the engaging means consists of the engaging ring and another engaging element, e.g., of the engaging ring and a engaging collar. Furthermore, the term xe2x80x9cengaging ringxe2x80x9d includes not only solid support rings having perfect support ring shapes but also split rings partially split by a slit.
The engaging rings are set within the range of 0.5 mmxe2x89xa6Wxe2x89xa61.2 mm in width W for the following reason. The engaging rings, as described previously, undergo repeated axial loads via the rollers (or the support rings) and the needle rollers. It is thus essential to provide the engaging rings with appropriate toughness for the sake of higher capacity for the axial loads and higher fatigue strengths. That is, the provision of appropriate toughness for the engaging rings disperses the axial loads imposed on the engaging rings, resulting in an improvement in the fatigue strengths of the engaging rings. Besides, in constant velocity universal joints of this kind, engaging rings are often attached to the rollers or the support rings as contracted/expanded in diameter. Therefore, the provision of appropriate toughness to the engaging rings is also desirable in terms of mountability. Furthermore, in favor of simplified fabrication processes, consideration is desirably given to the workability of the engaging rings. The setting of the engaging rings within the range of 0.5 mmxe2x89xa6Wxe2x89xa61.2 mm in width W can provide appropriate toughness for the engaging rings, so that they are improved in the fatigue strength against axial loads and enhanced in the mountability to the rollers or support rings at the same time. Here, the engaging rings also improve in workability.
Meanwhile, in favor of higher fatigue strength against the axial loads and enhanced fatigue life of the contact surfaces, it is desirable that the surfaces of the engaging rings be provided with appropriate hardness for excellent wear resistance. It is for this reason that the engaging rings are set within the range of HRC 43 to HRC 53 in surface hardness. The term xe2x80x9cHRCxe2x80x9d here represents C scale in Rockwell hardness. Surface hardnesses below HRC 43 cannot provide the contact surfaces with sufficient fatigue life. Surface hardnesses above HRC 53 cause a drop in toughness, which is unfavorable in views of fatigue strength against axial loads and in terms of mountability.
In the constitution described above, at least surface layers of the engaging rings may contain a structure in which spheroidized carbide is distributed into a martensite matrix. Here, the phrase xe2x80x9cat least surface layers of the engaging rings contain a structure in which spheroidized carbide is distributed into a martensite matrixxe2x80x9d covers such constitutions that only the surface layers contain the above-mentioned structure, and that the above-mentioned structure extends from the surfaces to the insides.
According to this constitution, at least the surface layers of the engaging rings are provided with the structure with martensite matrix containing spheroidized carbide. This yields a wear resistance higher than those of steels for general structure, thereby improving the contact surfaces in fatigue life.
The above-mentioned carbide consists mainly of Fe3C. The structure having such carbide distributed into its martensite matrix can be formed by providing at least the surface layers with carbon C as much as or more than its eutectic point (0.8% by weight or higher), and subjecting the same to hardening and tempering.
To be more specific, the engaging rings may be made of carbon tool steel, and the martensite matrix be provided with a spheroidized carbide content of 0.3-0.6% by weight. According to this constitution, the martensite matrix contains an appropriate amount of fine-spheroidized carbide, and therefore achieves higher wear resistance. In the meantime, the core portion is prevented from an excessive increase in hardness, and thus forms a structure of appropriate toughness. As a result, the contact surfaces of the engaging rings improve in fatigue life, as well as in the fatigue strength against axial loads. Moreover, since the engaging rings secure an appropriate toughness, they are also enhanced in the mountability to the rollers or support rings. Here, the martensite matrix is preferably limited to the range of 0.3-0.6% by weight in spheroidized carbide content. Spheroidized carbide contents below 0.3% by weight cannot produce the effect of improving the wear resistance sufficiently. In contrast, spheroidized carbide contents above 0.6% by weight can make the matrix so low in toughness as to fall short of the fatigue strength against axial loads and the mountability. The carbon tool steel may use SK3, SK4, SK5, SK6, and the like.
Otherwise, the engaging rings may be made of spring steel. According to this constitution, higher elastic limits can be achieved while maintaining high surface hardness. Therefore, the contact surfaces of the engaging rings improve in fatigue life as well as in the fatigue strength against axial loads. Moreover, the achievement of higher elastic limits further improves the engaging rings in mountability, which is also effective in automating the mounting process and thereby reducing the fabrication costs. The spring steel can be selected and used irrespective of type; an optimum one may be selected from among hot-forming spring steels and cold-forming spring steels in accordance with use conditions, joint size, and the like. For example, hot-forming spring steel SUP4 and the like may be used.
The engaging rings may also be made from a hard steel wire rod. Although slightly inferior in wear resistance as compared with the constitutions described above, this constitution provides higher elastic limits, and thereby disperses the axial loads imposed on the engaging ring. As a result, higher fatigue strengths against axial loads are obtained. Besides, hard steel wire rods are relatively inexpensive as well as effective at improving mountability. For example, the hard steel wire rod may use SWRH or the like.
In the above-described constitutions, the engaging rings are preferably attached to the rollers or the support rings with no play. The phrase xe2x80x9cwith no playxe2x80x9d here refers to a state in which the engaging rings are mounted to the rollers or the support rings at least with no radial play. Elimination of axial play as well as the radial play is preferable. According to this constitution, the no-play attachment of the engaging rings to the rollers or the support rings stabilizes the areas of action (the load points) of the axial loads the engaging rings receive from the rollers or the support rings. This results in enhanced fatigue strength against axial load. Besides, the suppression of the load-point fluctuations also improves the fatigue life of the contact surfaces between the engaging rings and the rollers or the support rings.
In addition, the other of the engaging means may be composed of a engaging collar formed integrally on a roller or a support rings so that assembling tolerance due to the attachment of a engaging ring to this portion is eliminated. As a result, the axial clearances from the engaging means on both sides to the roller or the support ring can be reduced by half. This can make the above-described effects more significant.
In the constitutions described above, many minute dimples may be formed randomly at least in the contact surfaces of the engaging means (engaging rings and/or engaging collars). The minute dimples formed in the contact surfaces function as oil sumps to promote the formation of oil films on the contact surfaces, improving the lubricity and enhancing the rolling fatigue life of the contact surfaces. For example, the minute dimples have a size of several tens of xcexcm or so, and a depth of 1 xcexcm or so. Grinding conditions to the contact surfaces can be changed to form minute dimples of arbitrary size, depth, and number. When it is difficult to form minute dimples selectively in the contact surfaces alone, minute dimples may also be formed in the vicinities of the contact surfaces or over the entire surfaces of the engaging rings and the rollers/support rings.
A solid lubrication coating may be formed at least on the contact surfaces of the engaging means (the engaging rings and/or the engaging collars), with a chemical conversion coating as an undercoating. Since the solid lubrication coating reduces the frictional resistance on the contact surfaces and improves the lubricity, the contact surfaces improve in fatigue life. The chemical conversion coating to be the undercoating is formed with the objective of increasing the solid lubrication coating in adhesion to the contact surfaces. Examples of the chemical conversion coating include a manganous phosphate coating, an iron phosphate coating, and a zinc phosphate coating. Examples of the solid lubrication coating include a molybdenum disulfide coating and a PTEE coating. In this connection, the effect after the treatment depends on the pre-treatment surface roughness of the contact surfaces (base-material surfaces). It is therefore desirable that the contact surfaces be previously finished with a surface roughness of Ra 0.2-0.8 for the sake of appropriate oil-sump functions. In the cases where the selective application of coating to the contact surfaces alone is difficult, the coating may also be applied to the vicinities of the contact surfaces, or over the entire surfaces of the engaging rings and the rollers/support rings.
Cold sulfurizing may be applied at least to the contact surfaces of the engaging means (the engaging rings and/or the engaging collars). Sulfurizing is a surface treating method for infiltrating sulfur to the surface of steel to generate iron sulfide. The application of sulfurizing reduces the frictional resistance on the surface; therefore, the surface improves in initial conformability for enhanced rolling fatigue life and stabilized NVH performances as well. Since the cold sulfurizing is performed under such a condition as 30-40xc2x0 C.xc3x9710-30 min., no hardness drop occurs in the surface hardened layers. The effect after the treatment depends on the pre-treatment surface roughness of the contact surfaces (base-material surfaces). Thus, it is desirable that the contact surfaces be previously finished with a surface roughness of Ra 0.2-0.8 for the sake of appropriate oil-sump functions. In the cases where the selective application of sulfurizing to the contact surfaces alone is difficult, the sulfurizing may also be applied to the vicinities of the contact surfaces, or over the entire surfaces of the engaging rings and the rollers/support rings.
Shot peening may be applied at least to the contact surfaces of the engaging means (the engaging rings and/or the engaging collars). With the conditions including the size of the shot particles, the speed of shot, and the amount of shot adjusted appropriately, minute dimples can be formed in the contact surfaces so that the minute dimples have the oil sump functions for improved lubricity. The application of the shot peening produces finer surface structures as well as causes residual compressive stress on the surfaces. Therefore, the shot peening is effective at improving the fatigue strength against axial loads and the fatigue life of the contact surfaces. In the cases where the selective application of shot peening to the contact surfaces alone is difficult, the shot peening may also be applied to the vicinities of the contact surfaces, or over the entire surfaces of the engaging rings and the rollers/support rings.
The constant velocity universal joints according to the present invention may use roller assemblies each including a roller to be guided by the roller guideways and the support ring mounted on the outer periphery of the trunnion to support the roller rotatably, wherein: the inner periphery of the support ring is shaped arcuate and convex in section; and the outer periphery of the trunnion is shaped straight in longitudinal section, and formed in cross section so as to make contact with the inner periphery of the support ring in a direction perpendicular to the axis of the joint and create a clearance with the inner periphery of the support ring in an axial direction of the joint. Otherwise, the constant velocity universal joints according to the present invention may use roller assemblies each including a roller to be guided by the roller guideways and a support ring fitted on the outer periphery of the trunnion to support the roller rotatably, wherein: the trunnion has a convex-spherical outer periphery; and the support ring has a cylindrical or conical inner periphery. Since the details of these constitutions are identical to those described previously, description thereof will be omitted.
According to the present invention, the engaging means, especially the engaging rings to be attached to the rollers/support rings, improve in the fatigue strength against axial loads and in the fatigue life of their contact surfaces. This makes it possible to provide a tripod type constant velocity universal joint of superior durability and strengths while maintaining its current dimensions, as well as to provide a tripod type constant velocity universal joint of more compact configuration while securing durability and strengths equivalent to or higher than those of existing products.